1. Field of the Invention
The present invention relates to a method and control system for shutdown of the internal combustion engine of a hybrid electric vehicle.
2. Description of Related Art
Evaporative emission control systems are widely used in conventional internal combustion engine (ICE) powered vehicles to reduce evaporated fuel, i.e., fuel vapor emissions from the fuel system to the atmosphere that is alleged to contribute to smog. In such evaporative emission control systems, fuel vapor from the fuel system and from a vapor storage canister are drawn into the engine via a vapor management valve (VMV). The amount of fuel vapor introduced into the intake manifold, and thus into the engine cylinders to be combusted, is proportional to how much the VMV is opened. To maintain stoichiometric air/fuel ratio in the engine cylinder that is optimum for minimizing tailpipe emissions, a typical VMV control strategy accounts for how much fuel in the engine cylinder is due to the VMV. In this way, the fuel control strategy can command the fuel injectors to deliver proportionately less fuel than would normally be scheduled if the VMV was not allowing fuel vapor into the engine cylinder.
In conventional tailpipe emissions control systems of ICE powered vehicles, an exhaust gas recirculation (EGR) valve is used to recirculate a portion of the hot exhaust gases back into the intake manifold, thereby diluting the air/fuel charge and lowering combustion temperatures to reduce the amount of NOx (oxides of nitrogen) that are created. The amount of exhaust gases recirculated into the intake manifold, and thus into the cylinders, is proportional to how much the EGR valve is opened. Though mostly made up of inert byproducts of the previous combustion event, the exhaust gases do partially contain some unburned fuel vapor.
In a hybrid electric vehicle (HEV), the ICE is periodically operated alone, or in conjunction with one or more electric motors, as instructed by a vehicle system controller (VSC). The VSC processes wheel torque requests from the driver via the accelerator pedal and determines how best to deliver that wheel torque from a minimal fuel consumption standpoint. For example, the VSC determines whether to use the electric motors only with the ICE xe2x80x9coffxe2x80x9d, the ICE xe2x80x9conxe2x80x9d with the electric motor(s) xe2x80x9coffxe2x80x9d, or to use a hybrid mode where both the electric motor(s) and the ICE are xe2x80x9conxe2x80x9d and contributing to the desired wheel torque. Thus, during operation of the HEV, the ICE is periodically started and stopped as instructed by the VSC during the course of a drive cycle.
During an engine shutdown in a HEV drive cycle, the VMV and EGR valve may be flowing at different rates depending on when the shutdown occurs, and thus may contribute fuel vapor amounts to the intake manifold that vary from one engine shutdown to the next. This, in turn, leads to inconsistent amounts of residual fuel vapor left in the intake manifold from one subsequent engine restart to the next. Because of the many engine shutdowns and starts in an HEV, it is desired to minimize the amount of tailpipe emissions during these events. However, with an inconsistent amount of residual fuel vapor, it becomes very difficult to deliver the proper amount of fuel through the injectors from one engine start to the next during the course of a drive cycle. Thus, tailpipe emissions may vary from one engine start to the next during a drive cycle.
The present invention provides, pursuant to one embodiment, a method and control system for shutdown of the ICE wherein a VMV of the evaporative emissions control system and an EGR valve of the tailpipe emission control system are disabled (shut off or closed) at the time of an engine shutdown. A specific embodiment of the invention involves ICE shutdowns that occur during the drive cycle of an HEV. The shutdown command is provided by a vehicle system controller to a controlled engine shutdown routine of an engine controller that, after closing of the valves, commands disabling of the engine fuel injectors to stop engine operation. The fuel injectors can be disabled in a ramp mode over time where individual fuel injectors are disabled in a calibratable sequence. The vehicle system controller and the engine controller can reside in one or separate control modules.
In an illustrative embodiment of the invention, the VMV and EGR valves of the ICE are disabled (shut off) at the time the engine shutdown command is provided to a controlled engine shutdown routine of the engine controller, and prior to the fuel injectors being disabled by the controlled engine shutdown routine. After the fuel injectors are disabled, the vehicle system controller instructs an electric motor of the HEV to spin the internal combustion engine for a predetermined time, thereby pushing residual fuel out of the intake manifold into the catalytic converter to be converted. The controlled engine shutdown routine commands the ignition system to continue firing the spark plugs during the time periods that the fuel injectors are being disabled and the engine is being spun by the electric motor until the engine speed drops below a predetermined value.
The present invention is advantageous to provide a more consistent amount of residual fuel vapor in the intake manifold of an ICE at each engine shutdown. The present invention is further advantageous to control shutdown of an ICE of an HEV in a manner that provides a more consistent amount of residual fuel vapor in the intake manifold of the ICE at each engine shutdown and thus at each engine restart as the engine is cycled between the xe2x80x9conxe2x80x9d state and xe2x80x9coffxe2x80x9d state during operation of the HEV. This is accomplished by eliminating contributions of fuel vapor to the intake manifold from the VMV and EGR valve at engine shutdown. The engine thereby can be restarted with a more consistent amount of fuel in the intake manifold so that the air/fuel ratio at engine restart can be controlled with minimal variation from start to start and therefore help reduce variability of tailpipe emissions at engine restarts during the drive cycle operation of an HEV.
The above objects and advantages of the present invention will become more readily apparent from the following description taken with the following drawings.